1. Field of the Invention
The invention relates to an assembly unit.
2. Background and Relevant Art
In many branches of industry, components or assembly parts which have to be fastened or mounted on a basic structure are already provided with fastening means in a captive manner by the manufacturer such that such an assembly unit facilitates the subsequent assembly of the assembly part on a basic structure. The assembly parts or a fastening flange which is present thereon comprise at least one bore which penetrates them and which is penetrated by the shank of a fastening means, for example of a screw. The fastening means additionally comprises a head, which protrudes radially beyond the shank and overlaps the bore edge of the bore in a radial manner, and, on the end remote from the head of the fastening means, a fixing portion which is realized, for example, as a thread. For fixing the assembly part on the basic structure, the fixing portion engages in a fixing bore in the basic structure, for example a threaded bore. The fastening means is held in a captive manner in the bore so as to be movable between a first axial position and a second axial position. In the first axial position the shank protrudes from the assembly surface of the assembly part remote from the head at a maximum protrusion which comprises at least part of the fixing portion. In the second axial position the shank preferably does not protrude from the assembly surface or protrudes at a protrusion that is reduced compared to the first axial position. During the course of the assembly, the assembly part can consequently be positioned either up to a small spacing which corresponds to the reduced protrusion or, where there is no protrusion, can be positioned on the basic structure so as to form mutual surface contact.
DE 198 02 497 A1 makes known an assembly unit where the fastening means is held so as to be axially movable in the bore of the assembly part by means of a compression spring which is formed from helically wound spring wire and encompasses the shank of the fastening means. The compression spring is fixed with its one end, close to the head, on the shank of the fastening means and with its other end, remote from the head, on the assembly part. Disadvantageous in the case of the known assembly unit is, in particular, that the depth of penetration of the fastening means or of its fixing portion into a counter bore of the basic structure, in the case of a screw the depth of penetration thereof, that is necessary for sufficient fixing has an effect on the axial space required for the compression spring in the final assembly state in which the assembly part is clamped with the basic structure. The greater namely the depth of penetration of the fixing portion, the greater the number of spring wire windings at a given pitch of said spring wire windings. As the spring wire windings in the maximum shortened state of the compression spring abut against one another axially, the number of windings is added to a corresponding length. If, for example, the spring in its maximum shortened state is to be arranged fully inside the bore of the assembly part, an assembly part with a corresponding bore length or thickness is necessary.
Proceeding from here, it is the object of the invention to propose a correspondingly improved assembly unit.